Running out of in-focus with 12" dob

[hurricane (Andrew)]

Hi all,

I recently bought a Philips SPC 900NC webcam, and had it hooked up to my 12" GSO dob last night with the adapter (bought from Peter at tan14).
I couldn't get Mars or Saturn focused however as it would run out of in-focus (if that's what it is called).

The eyepiece focuser was all the way in it could go.

Do I need a focal reducer to use the webcam? I have a 2x Barlow which I tried using too, but couldnt see any results with that either, would that be because the magnification would be too high?

[mswhin63 (Malcolm)]

For planetary imagery a DOB is difficult and will need high magnifacation to get a good result. You need to find a way of getting the CCD element closer to the focal point to acheive focus, I have only used another web cam and managed to get only 1mm of travel (lucky me) but I expect with such a fine tolerance it will be difficult al-be-it not impossible. I have no XP on the SPC 900.

It is strange though that the barlow also experience the same problem, I usually find a lot more available focus with my barlow but I have only tested mine, I though have difficulty tracking the DOB for smaller planets and have only succeded in non barlow shots as the planet is difficult to align.

If you have a collapsable DOB, just collapse it a bit at a time, if not you can try to re-colimnate the primary mirror moving it forward. I did this as well and got an extra 5mm.

On another note you may get a bit of noise around the planet which is normal, this is why images are processed to remove this noise that could look like an out of focus picture.

Finally if the above doesn't work low profile focuser or cutting the tube (argh) maybe the only way.

[Scorp56 (Peter)]

Hi Andrew.
I have a new GSO 8" Dob and SPC 900NC also. I do not have the adapter yet, but plan to do some imaging. This "focus" issue may also be a problem for me. Is it about getting the CCD sensor at prime focus?

Maybe some more experienced members can offer advice.

Cheers

Peter

[Insane Climber (Jason)]

Hello
Yes often cameras will not get enough in focus. however the use of a barlow will allow you to achieve focus, as the barlow puts the focal plane further back (back focus). I would suggest that your problem is you don't know where abouts the focus point is. and trying to get tiny mars into view is difficult, especially with the barlow. try focusing on a distant tree or building during the daytime and mark where focus is. when centering mars at night use a high power eyepiece to center the object, then change it out for the cam and bring the focus to the point you marked earlier.

Good luck and have fun

[stephenb (Stephen)]

This is a very common experience for many and there are two very common fixes for this...

1. You can purchase a GSO low profile focuser (approx. $129) from BinTel or Andrews. Only takes 4 screws to attach. Once you have the low profile focuser, you may need an extension tube for your eyepieces when doing visual observing.

2. Move the primary mirror up the tube by about 1 inch or so. More members here will give you details on how to do this.

[bmitchell82 (Brendan)]

Just a quick one if you are thinking about doing any astrophotography (apart from planetary) don't get the gso focuser it will not lift the weight of the camera and associated pieces.! how do i know, ive been there.!

[mswhin63 (Malcolm)]

Thanks for the info Brendan, was considering it myself, may best with JMI or other instead, save the pennies.

[monoxide]

yep, the GSO low profile focuser is a pile of faeces.

if using the barlow makes it too hard to get focus try unscrewing the actual barlow lens part off the bottom of it and screwing that straight onto the filter thread of the webcam adapter. that will drop the magnification but may still give you enough to reach focus